1. Technical Field
The present invention relates to a line head, an image forming apparatus using the line head, and a method for adjusting the position of the line head.
2. Related Art
In general, a toner image former using an electrophotography technique comprises a photosensitive member which has, on an outer peripheral face thereof, a photosensitive layer which serves as an image carrier; a charger for uniformly charging the outer peripheral face of the photosensitive member; an exposer which selectively exposes the outer peripheral face uniformly charged by the charger, to thus produce an electrostatic latent image; and a developer for imparting toner serving as a developing agent to the electrostatic latent image produced by the exposer, thereby transforming the electrostatic latent image into a visible image (a toner image).
In a tandem-type image forming apparatus for producing a color image, the toner image former is disposed in numbers (e.g., four) in relation to an intermediate transfer belt. Respective toner images produced on the photosensitive member by toner image former of single colors are sequentially transferred onto the intermediate transfer belt, whereby toner images of a plurality of colors (e.g., yellow, cyan, magenta, black) are overprinted one on the other on the intermediate transfer belt, to thus produce a color image on the intermediate transfer belt.
In such a tandem-type image forming apparatus, as a light emitter in the exposer, a line head using an LED or an organic EL element is used. When a process cartridge removably attached to the line head is adopted, difficulty is encountered in direct positioning of the shaft of the photosensitive member with respect to a main body of the image forming apparatus if the removable attachment direction is parallel to the direction of the shaft of the photosensitive member.
When the structure of such a removable process cartridge is adopted, the parallelism of the shaft of each of the photosensitive members varies from a cartridge of one color to a cartridge of another color. In the tandem color image forming apparatus, even when scan lines to be written are accurately parallel to each other, if the shaft of the photosensitive member is inclined, the scan lines become nonparallel at a point in time when the image written in the photosensitive member is developed and transferred to an intermediate transfer belt. Namely, skewing arises, which in turn creates misregistration of colors.
In addition, variations in the reference position where the cartridge is attached to the main body also cause an error in the parallelism of each of the color cartridges. The parallelism of the photosensitive member attached to the cartridge is impaired by such an error in parallelism, thereby causing a skew in the same manner as mentioned above. Since the misregistration of colors causes deterioration of image quality, measures must be taken to prevent occurrence of skewing.
However, in the tandem color-image forming apparatus using the line head, it is very difficult to maintain parallelism between respective scan lines to be transferred to an intermediate transfer belt by only an improvement in mechanical accuracy of individual sections. To cope with such a problem, Japanese Patent Publication No. 4-166824A (JP-A-4-166824) discloses a mechanism for adjusting positions of the line head in three axial directions as well as adjusting attitudes of the line head around three axes.
Japanese Patent Publication No. 10-73980A (JP-A-10-73980) discloses a tandem-type image forming apparatus performing a method for dividing writing operation of a line head into several sub-operations and sequentially delaying writing timings of the writing operations to thus electrically correct skews of respective colors. Japanese Patent Publication No. 10-16294A (JP-A-10-16294) discloses a mechanism which adjusts both ends of a member supporting an LED array in a secondary scanning direction with respect to a graded-index lens array (Selfoc Lens Array). Japanese Patent Publication No. 2002-337392A (JP-A-2002-337392) discloses a mechanism for adjusting the position of a substrate provided with a light emitter with respect to a main body of the line head.
Incidentally, in a line head using, as a light emitter, an organic EL element or an LED, the line head is positioned by inserting, into a cartridge, reference pins provided on the respective ends of the line head. In the case of a line head using an LED, the line head may be attached to the main body of the image forming apparatus. A line head using an LED generally employs a plurality of LED array chips, each of which is formed by integrating tens to 200 light emitting elements, and the LED array chips are mounted on a substrate, thereby producing a head of predetermined length.
In the line head of such a configuration, a line (scan line) formed by arranging in a row the light emitting elements of the respective LED array chips is curved by a positional error caused at the time of mounting of the LED array chips. In order to focus the light beams emitting from the LED array chips into an image on a plane to be scanned (hereinafter simply called a “scanned plane”), a rod lens array is provided. This rod lens array exhibits the action of projecting, in an erecting manner, the light beams from the light source onto the scanned plane at the same magnification.
Accordingly, the layout of the light sources is projected, without modification, onto the scanned plane even when errors are present. When the LED array chips are mounted in a curved manner as mentioned above, the arrangement of an image spot formed by converging light beams from the LED array chips is also curved in the same manner. In short, an image is displaced in the secondary scanning direction.
Correction of an error in the LED array chips in the secondary scanning direction will be described. In FIGS. 18A and 18B, reference numeral 69 designates reference pins provided on both ends of the line head in the primary scanning direction thereof; and reference symbol CL designates a line connecting reference pins provided on respective ends of the line head and corresponds to a center line of the line head in the primary scanning direction. Reference numerals 76a to 76g designate LED array chips which are to be arranged in numbers in the primary scanning direction of the line head. Each of the LED array chips is provided with a plurality of LEDs.
When the arrangement of each of the LED array chips in a secondary scanning direction has a positional error, a correction value is determined from the amount of displacement of a reference line, which is defined by connecting both end points of the head of that value, by reference to the pixel in the center of the LED array chip or an average value of positional data at both ends of the LED array chip as shown in FIG. 18B. However, when the positional error is corrected by such a method, there may arise a case that a step “da” at the boundary between the LED array chips is extended further as shown in FIG. 18A. Particularly, when the amount of correction in the secondary scanning direction can assume only a discrete value, the step tends to be extended further.
Even when the line head has been accurately adjusted in three axial directions by the adjustment mechanism, such as that described in JP-A-4-166824, if the inclination of an exposure position (a scan line) with respect to the reference (e.g., a reference pin) used for attachment of the line head varies from one line head to another, there arises a problem of the necessity for performing re-adjustment at the time of replacement of the line head.
As described in JP-A-10-73980, the inclination can be corrected (skew correction control) by electrically shifting a writing timing. However, when a skew is corrected by reference to only an initial value, the initial value is changed at the time of replacement of the line head, and hence there arises a necessity for correcting a skew correction value as in the case of JP-A-4-166824.
In order to avoid such complicated processing, a sensor may be provided on each end of the intermediate transfer member, to thus detect the position (inclination) of each of color images and automatically correct the detected inclination. However, a sensor for this purpose is required to be placed in at least two positions; namely, both ends of the intermediate transfer member. This leads to a problem of a complicated structure and a cost hike.
In order to control such skew correction, circuitry of a considerable scale, such as circuitry for changing the sequence of transfer of image data, is required, which in turn raises a problem of excessive cost. Moreover, even in a case where skew correction is performed, when variations in the inclination of the location, where the line head is to be exposed, are great, the range of skew correction control is increased. This raises a problem of an increase in the volume of temporary storage memory required to perform skew correction.
The technique described in JP-A-10-16294 is for adjusting the member supporting an LED with respect to the lens array and not for adjusting the inclination of an image forming position with respect to the line head mount reference section. Likewise, the technique described in JP-2002-337392 is to adjust the substrate having light emitters mounted thereon in the secondary scanning direction. The technique also describes adjustment of the position of the substrate with respect to a lens array, and JP-2002-337392 does not refer at all to adjustment of the line head attachment reference position. Therefore, the techniques described in JP-A-10-16294 and JP-2002-337392 encounter a problem of difficulty in preventing occurrence of skewing and misregistration of colors. Moreover, the techniques described in Patent JP-A-10-16294 and JP-2002-337392 relate to a mechanism for simultaneously adjusting an absolute value of the position in the secondary scanning direction and an inclination. Hence, there arises a problem of involvement of a complicated structure and laborious adjustment work.
When a large step exists in a boundary between the LED array chips as shown in FIG. 18A, a discrepancy between images arises along the step. Accordingly, when a grayscale image is expressed in halftone dots or oblique lines, a difference in density arises along the step. When the step is large, there arises a problem of a step appearing in an image which should originally be straight.